Predictions of the 21 cm global signal in the JWST and ALMA era

Abstract

ABSTRACT We calculate the redshift evolution of the global 21 cm signal in the first billion years using an advanced semi-analytical galaxy formation model delphi. Employing only two redshift- and mass-independent free parameters, our model predicts galaxy populations in accord with data from both the JWST and the Atacama Large Millimetre Array (ALMA) at z ∼ 5–12. In addition to this ‘fiducial’ model, which fully incorporates the impact of dust attenuation, we also explore an unphysical ‘maximal’ model wherein galaxies can convert a 100 per cent of their gas into stars instantaneously (and supernova feedback is ignored) required to explain JWST data at z > =13. We also explore a wide range of values for our 21 cm parameters that include the impact of X-ray heating (fX,h = 0.02–2.0) and the escape fraction of Lyman Alpha photons (fα = 0.01–1.0). Our key findings are (i) the fiducial model predicts a global 21 cm signal, which reaches a minimum brightness temperature of Tb, min ∼ −215 mK at a redshift zmin ∼ 14; (ii) since the impact of dust on galaxy properties only becomes relevant at z < = 8, dust does not have a sensible impact on the global 21 cm signal; (iii) the ‘maximal’ model predicts Tb, min = −210 mK as early as zmin ∼ 18; and (iv) galaxy formation and 21 cm parameters have a degenerate impact on the global 21 cm signal. A combination of the minimum temperature and its redshift will therefore be crucial in constraining galaxy formation parameters and their coupling to the 21 cm signal at these early epochs.